Retardation plates are generally fabricated either from mineral monocrystals, which are expensive and poorly adapted to large areas, or else from organic polymer materials, such as polycarbonate. The thermal and photochemical stability of organic wave plates is low. Unfortunately, in certain applications, the use of a wave plate requires good stability relative to temperature or to a high flux of photons.
Furthermore, wave plates based on organic materials require sufficient thickness to form a plate having the desired optical retardation, for example, a quarterwave plate to generate optical retardation equal to λ/4, or a halfwave plate for retardation of λ/2, or indeed a wave plate for retardation equal to λ. The thickness of an organic wave plate generally has the effect of reducing the transmission coefficient of the wave plate, in particular in the UV range shorter than 350 nm.
Various methods have been proposed for replacing the organic polymer materials of wave plates with inorganic materials for the purpose of improving the technical stability of such wave plates.
The following documents (I. Hodgekinson and Qi hong Wu, “Serial bideposition of anisotropic thin films with enhanced linear birefringence”, Appl. Optics, Vol. 38, No. 16, pp. 3621-3625; A. C. van Popta et al., “Birefringence enhancement in annealed TiO2 thin films”, Journ. Appl. Phys. 102, 013517, 2007; O. L. Mustens et al., “Epitaxial growth of aligned semiconductor nanowire metamaterials for photonic applications”, Adv. Func. Mater. 2008, 18, pp. 1039-1046) are known, which describe the epitaxial growth of inorganic nanorods on an inclined substrate by chemical vapor deposition (CVD). The technique of epitaxy is known in the field of semiconductors. Nevertheless, epitaxial growth has the major drawback of causing nanorods to grow that are not parallel to the surface of the substrate. As a result, the geometrical anisotropy is not fully devoted to birefringence in the plane of the surface of the substrate. Transmitted light presents complex optical retardation behavior that depends strongly on the angle of incidence of the light. The birefringent films of nanorods that are obtained in that way generally present strong absorption or diffusion in the visible spectrum range. Furthermore, that method is difficult to apply industrially to producing film on large areas because of limits in terms both of substrate surface area and of cost.
Other techniques rely on using colloidal suspensions of anisotropic nanoparticles. The document (M. Mittal and E. M. Furst “Electrical field-directed convective assembly of ellipsoidal colloidal particles to create optically and mechanically anisotropic thin films”, Adv. Func. Mater, 2009, 19, pp. 3271-3278) describes an assembly of particles directed by an electric field, which requires a complex deposition technique to be performed together with the application of a strong electric field.
The document (M. Mittal et al., “Flow-directed assembly of nanostructured thin films from suspensions of anisotropic titanium particles”, Nanoscale 2010, 2, pp. 2237-2243) describes assembly directed by a flow of TiO2 nanoparticles that is easy to perform, but in which orientation order is difficult to control. That technique does not make it possible to orient nanoparticles on a large area. Imperfect orientation of TiO2 nanoparticles has the drawback of producing strong diffusion and of reducing the birefringent properties of the film.
The document (H. Miyata et al., “Remarkable birefringence in a TiO2—SiO2 composite film with an aligned mesoporous structure”, J. Am. Chem. Soc. 2011, 133, pp. 13539-13544) describes fabricating a composite film having an aligned mesoporous structure. The film presents birefringence of 0.06 and good optical transmission due to the small size of the pores (a few nanometers). Nevertheless, the film presents cracking and a top layer that is not aligned. The thickness of the film appears to be limited to 200 nanometers (nm), which is insufficient for fabricating a quarterwave plate or a halfwave plate in the visible.
Furthermore, patent document EP 1 715 365_A1 describes an optical component for liquid crystal display that includes a birefringent thin layer comprising anisotropic mineral particles and a polymer resin.
Document JP-A-2009-104152 describes a birefringent thin film having inorganic nanoparticles and an organic binder.
Finally, document JP-A-2012-032623 describes a method of fabricating a film made up of a polymer resin and oriented mineral particles.
Nevertheless, those various techniques have the drawbacks of producing a film with low birefringence, high diffusion, and/or high absorption in the visible spectrum range, or indeed of presenting defects of uniformity, cracking, or limits in terms of dimensions (thickness, area). Furthermore, fabrication costs make those techniques difficult to apply industrially.